- Enzymatic reactions occur in a crowded and confined environment in vivo, containing proteins, RNA and DNA. Previous reports have shown that interactions between macromolecules, and reactions rates differ significantly between crowded environments and dilute buffers. However, the direct effect of crowding on the level of high-resolution structures of macromolecules has not been extensively analyzed and is not well understood. Here we analyze the effect of macromolecular crowding on structure and function of the human translation initiation factors eIF4A, a two-domain DEAD-Box helicase, the HEAT-1 domain of eIF4G, and their complex. We find that crowding enhances the ATPase activity of eIF4A, which correlates with a shift to a more compact structure as revealed with small-angle X-ray scattering. However, the individual domains of eIF4A, or the eIF4G-HEAT-1 domain alone show little structural changes due to crowding except for flexible regions. Thus, the effect of macromolecular crowding on activity and structure need to be taken into account when evaluating enzyme activities and structures of multidomain proteins, proteins with flexible regions, or protein complexes obtained by X-ray crystallography, NMR, or other structural methods.